Active Photosensing Pixel

ABSTRACT

An active photosensing pixel is disclosed, in which a two-terminal photosensing transistor has a first terminal coupled to a first node, a second terminal coupled to a selection line and a control terminal connected to the second terminal. A driving transistor has a first terminal coupled to a first reference voltage, a second terminal coupled to an output line and a control terminal connected to the first node. A reset capacitor has a first terminal connected to the control terminal of the two-terminal photosensing transistor, and a second terminal connected to the first node.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority of Taiwan Patent Application No.099119406, filed on Jun. 15, 2010, the entirety of which is incorporatedby reference herein.

BACKGROUND

1. Field of the Disclosure

The present disclosure relates to a display, and in particular relatesto a photosensing pixel display.

2. Description of the Related Art

Lately, E-books have been developed and commercialized. One feasibledisplay configuration for E-books, adopts a thin film transistor liquidcrystal display (TFT-LCD) thereto. In other words, the E-book displaysimages by using electronic components, such as a TFT or photosensingdevice, disposed on a backplane thereof. E-books must have the abilityto sense a photo so that marking on a screen by touching the screen maybe accomplished. As an example, however, for an E-book with aphotosensing function, since the photosensing device is installed underthe backplane, light transmittance therethrough is decreased. Thus,E-Books needs amount of time for exposure of marking on a screenthereof.

An active photosensing pixel which can quickly mark on screen of E-booksis desired.

BRIEF SUMMARY

A detailed description is given in the following embodiments withreference to the accompanying drawings.

An embodiment discloses an active photosensing pixel, comprising atwo-terminal photosensing transistor, a driving transistor and a resetcapacitor. The two-terminal photosensing transistor has a first terminalcoupled to a first node, a second terminal coupled to a selection lineand a control terminal connected to the second terminal. The drivingtransistor has a first terminal coupled to a first reference voltage, asecond terminal coupled to an output line and the control terminalconnected to the first node. The reset capacitor has a first terminalconnected to the control terminal of the two-terminal photosensingtransistor, and a second terminal connected to the first node.

An embodiment discloses another active photosensing array, comprising aplurality of selection lines, output lines and active photosensingpixels. Each active photosensing pixel comprises a two-terminalphotosensing transistor, a driving transistor and a reset capacitor. Thetwo-terminal photosensing transistor has a first terminal coupled to afirst node, a second terminal coupled to a corresponding selection lineand a control terminal connected to the second terminal. The drivingtransistor has a first terminal coupled to a first reference voltage, asecond terminal coupled to a corresponding output line and the controlterminal connected to the first node. The reset capacitor has a firstterminal connected to the control terminal of the two-terminalphotosensing transistor, and a second terminal connected to the firstnode.

An embodiment further discloses a photosensing method for an activephotosensing pixel, wherein the active photosensing pixel comprises atwo-terminal photosensing transistor having a first terminal coupled toa first node, a second terminal coupled to a selection line and acontrol terminal connected to the second terminal, a driving transistorhaving a first terminal coupled to a first reference voltage, a secondterminal coupled to an output line and a control terminal connected tothe first node and the reset capacitor, having a first terminalconnected to the control terminal of the two-terminal photosensingtransistor, and a second terminal connected to the first node. Thephotosensing method includes the following steps. First, a first voltageis provided to the selection line, during an exposure and readout cycle,such that the two-terminal photosensing transistor is served as a diode.Next, a diode current is generated to charge the first node when thetwo-terminal transistor receives an incident light, such that thedriving transistor is turned on according to a voltage level of thefirst node to produce an output current to the selection line.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure can be more fully understood by reading the subsequentdetailed description and examples with references made to theaccompanying drawings, wherein:

FIG. 1A is a schematic view showing an embodiment of the first operationmode for the two-terminal photosensing transistor;

FIG. 1B is an embodiment of the relationship of the photosensing currentI_(photo) and the voltage V_(N1) at the first terminal of thetwo-terminal photosensing transistor in the first operation mode;

FIG. 2A is a schematic view showing an embodiment of the secondoperation mode for the two-terminal photosensing transistor;

FIG. 2B is an embodiment of the relationship of the diode currentI_(diode) and the voltage V_(N2) at the second terminal of thetwo-terminal photosensing transistor in the second operation mode;

FIG. 3 is a schematic view showing an embodiment of the activephotosensing pixel;

FIG. 4 is a sequence diagram showing an embodiment of the selectionlines and the voltage waveform of the first node;

FIG. 5 is a schematic view showing another embodiment of the activephotosensing pixel;

FIG. 6 is a schematic view showing an embodiment of the activephotosensing array and a sequence diagram of the corresponding selectionlines;

FIG. 7 is a schematic view showing an embodiment of the display with anactive photosensing array and a sequence diagram of the correspondingselection lines.

DETAILED DESCRIPTION

The embodiment will be explained as follows. The following descriptionis made for the purpose of illustrating the embodiment and should not betaken in a limiting sense. The scope of the invention is best determinedby reference to the appended claims.

FIG. 1A is a schematic view showing an embodiment of the two-terminalsensing transistor in a first operation mode. In this embodiment, thetwo-terminal photosensing transistor (written as Q₁ hereinafter) is anN-type a-Si:H TFT, but is not limited thereto. The two-terminalphotosensing transistor Q₁ includes the first terminal N₁, the secondterminal N₂ and a control terminal. Note that the control terminal ofthe two-terminal photosensing transistor Q₁ is connected to the secondterminal N₂, wherein, it forms two terminals of the two-terminalphotosensing transistor. That is, the first terminal N₁ and the controlterminal are connected to the second terminal N₂. In the first operationmode, the high voltage V_(H) is applied to the first terminal of thetwo-terminal photosensing transistor Q₁ and the low voltage V_(L) isapplied to the second terminal N₂. The two-terminal photosensingtransistor Q₁ produces the photosensing current I_(photo) from the firstterminal N₁ to the second terminal N₂ when the two-terminal photosensingtransistor Q₁ receives the incident light hv in the first operationmode. Generally speaking, the intensity of the photosensing currentI_(photo) is determined by the surface dimensions of the semiconductorlayer and material of the two-terminal photosensing transistor Q₁.Moreover, the intensity of the photosensing current I_(photo) can alsobe determined by the intensity of the incident light hv. In other words,the stronger the incident light hv is, the greater the photosensingcurrent I_(photo) will be. Therefore, the two-terminal photosensingtransistor Q₁ is served as a photosensitive resistor in the firstoperation mode. For other embodiments, the two-terminal photosensingtransistor Q₁ can be a P-type a-Si:H TFT, but is not limited thereto. Inother embodiments, the two-terminal photosensing transistor Q₁ may bebipolar junction transistor (BJT) or other switching devices.

FIG. 1B is, during the first operation mode, the relationship of thephotosensing current I_(photo) and the voltage V_(N1) at the firstterminal of the two-terminal photosensing transistor Q₁. As shown inFIG. 1B, the magnitude of the photosensing current I′_(photo) is zero(also named cut-off region) when there is no incident light hv (the lineconnected by diamond points as shown). On the contrary, when thetwo-terminal photosensing transistor Q₁ receives incident light hv(indicated as the line that connected by the square points), thephotosensing current I_(photo), which is produced by the two-terminalphotosensing transistor Q₁, will linearly increase, before slowing downby degrees (known as the triode region), and finally, becoming saturated(known as the saturation region). Hence, the above description of thetwo-terminal photosensing transistor Q₁ is similar to the generalfield-effect transistor (FET). In one embodiment, the photosensingcurrent I_(photo) is around 7.5E−09A if the two-terminal photosensingtransistor Q₁ receives the incident light and the level of the firstterminal N₁ is 16V, and the photosensing current I′_(photo) is around 0Aif the two-terminal photosensing transistor Q₁ receives no incidentlight and the level of the first terminal N₁ is 16V. It can bedetermined whether the two-terminal photosensing transistor Q₁ hasreceived the incident light by detecting the photosensing currentI_(photo) during the first operation mode.

FIG. 2A is a schematic view showing an embodiment of the two-terminalphotosensing transistor Q₁ under the second operation mode. Similar withFIG. 1A, the control terminal of the two-terminal photosensingtransistor Q₁ is also connected to the second terminal N₂. Compared withthe first operation mode, a low voltage V_(L) is applied to the firstterminal N₁ of the two-terminal photosensing transistor Q₁ and a highvoltage V_(H) is applied to the second terminal N₂ for the secondoperation mode. Because both the control terminal of the two-terminalphotosensing transistor Q₁ and the second terminal N₂ are coupled to thehigh voltage V_(H) (MOS diode), the two-terminal photosensing transistorQ₁ is served as a diode in the second operation mode and produces thediode current I_(diode) (i.e. forward conducting current) through thesecond terminal N₂ to the first terminal N₁.

FIG. 2B shows the relationship between the diode current I_(diode) andthe voltage V_(N2) under the second terminal of the two-terminalphotosensing transistor in the second operation mode. Similar with thegeneral diode, the diode current I_(diode) of the two-terminalphotosensing transistor Q₁ is zero at the beginning, and thenexponentially increases after the two-terminal photosensing transistorQ₁ is turned on. After the two-terminal photosensing transistor Q₁ isconducted (V_(N2)>10V), whether the two-terminal photosensing transistorQ₁ receives or doesn't receive the incident light hv, will not changethe existence of the diode current I_(diode), flowing through the secondterminal N₂ to the first terminal N₁. It should be noted that thephotosensing current I_(photo) with incident light hv (the square pointas shown) is bigger than that of the current I′_(photo) without incidentlight hv (the diamond point as shown). In one embodiment, the diodecurrent I_(diode) is around 1.0E−09A if the two-terminal photosensingtransistor Q₁ receives the incident light and the second terminal V_(N2)is 15V. Conversely, if the two-terminal photosensing transistor Q₁ hasnot received incident light and the second terminal V_(N2) is at 15V,the diode current I′_(diode) will be around 0.5E−09A. Therefore, thesecond operation mode has two functions: the first is to determinewhether the two-terminal photosensing transistor Q₁ has received theincident light by detecting/determining the magnitude of the diodecurrent; and the second, is to discharge the second terminal N₂ by thediode current. Generally speaking, because the magnitude of the diodecurrent I_(diode) is much bigger than that of the photosensing currentI_(photo) (around 1.0E+03˜1.0E+04 units), the process of discharging thesecond terminal N₂ by the diode current in the second operation mode iscomparatively faster than that of charging the first terminal N₁ fromthe photosensing current in the first operation mode.

FIG. 3 is a schematic view showing the active photosensing pixel in oneembodiment. In this embodiment, the active photosensing pixel P₂₂comprises a two-terminal photosensing transistor Q₁, a drivingtransistor Q₂ and a reset capacitor C_(reset). The active photosensingpixel P₂₂ is coupled between the selection line Sel_2 and the outputline Out_2 perpendicular to the selection line Sel_2.

In the FIG. 3, the two-terminal photosensing transistor Q₁ has a firstterminal N₁ coupled to the first node X₁, a second terminal N₂ coupledto the selection line Sel_2 and a control terminal connected to thesecond terminal N₂. The driving transistor Q₂ has a first terminalcoupled to the first reference voltage V_(ref1), a second terminalcoupled to the output line Out_2 and the control terminal connected tothe first node X₁. The capacitor C_(reset) has a first terminal,connected to the control terminal of the two-terminal photosensingtransistor Q₁ and a second terminal connected to the first node X₁.

Following, is a description of the photosensing measure. As the sequencediagram of the selection lines and the waveform of the first node X₁shows in FIG. 4, the waveform of the first node X₁ comprises twosituations for the active photosensing pixel P₂₂: one is receiving theincident light hv; and the other is not receiving the incident light hv.In FIG. 4, the solid lines represent the sequence diagram of theselection line Sel_2 and Sel_3, respectively. The break line representsthe voltage waveform of the first node V_(X1) when the activephotosensing pixel P₂₂ has received the incident light hv, and thedotted line shows the voltage waveform of the first node V′_(X1) whenthe active photosensing pixel P₂₂ has not received incident light.

The following is a discussion about the operation mode of thetwo-terminal photosensing transistor. During the first cycle T₁ (knownas the exposure and readout cycle) the selection line Sel_2 is pulled upto a high voltage (such as a high driving voltage V_(GH)) which ishigher than that of the first node X₁, and the two-terminal photosensingtransistor Q₁ is served as a diode and produces the diode currentI_(diode) according to the incident light hv and further charges thefirst node X₁. For example, when the active photosensing pixel P₂₂receives the incident light hv, the two-terminal photosensing transistorQ₁ will produce a diode current I_(diode) according to the incidentlight hv, and further charge the first node X₁ to a high voltage V_(X1).When the voltage V_(X1) is higher than that of the threshold level ofthe driving transistor Q₂, the latter will be conducted by the voltageV_(X1) and produce an output current to the output line. Out_2: hence,the first cycle T₁ is also a readout cycle. On the other hand, duringthe first cycle T₁, if the level of selection line Sel_2 is pulled up tobe higher than that of the first node X₁ (such as a high driving voltageV_(GH)) and no incident light irradiates on the active photosensingpixel P₂₂, the first node X₁ will be charged to the high voltage V′_(X1)through the diode current I′_(diode) of the two-terminal photosensingtransistor Q₁. From the description above, it is clear that the secondoperation mode of the two-terminal photosensing transistor Q₁ has beenemployed. Note that when the incident light irradiates on thetwo-terminal photosensing transistor Q₁, the intensity of the diodecurrent I_(diode) is greater than that of the diode current I′_(diode)which has no incident light irradiating thereon, and then thetwo-terminal photosensing transistor Q₁ here is served as a diode.Therefore, designing the threshold level of the driving transistor Q₂between the V_(X1) and V′_(X1), that corresponds to the diode currentI_(diode) of the incident light and the diode current I′_(diode) havingno incident light, respectively, enables the driving transistor Q₂ to beable to be conducted by the diode current I_(diode) (having incidentlight) other than by the diode current I′_(diode) (not having incidentlight), so that whether the incident light hv irradiates on the activephotosensing pixel P₂₂ may be determined. Namely, the driving transistorQ₂ can be turned on when the voltage on the first node X₁ is V_(X1), andcan not be turned on when the voltage on the first node X₁ is V′_(X1).

During the second cycle T₂ (normally named reset cycle) the level ofselection line Sel_2 is pulled to be lower than that of that of thelevel of X₁ (such as the low driving voltage, V_(GL)), such that thereset capacitor C_(reset) will reset the first node voltage V_(X1), dueto the capacitive coupling effect, to turn off the driving transistorQ₂.

For the second cycle T₂, since the driving transistor Q₂ is notconducted, the two-terminal photosensing transistor Q₁ does not producethe diode current, even though the incident light hv is irradiatedthereupon. Note that the resetting of the voltage V_(X1) at the firstnode X₁ is completed by means of resetting the reset capacitor C_(reset)in this embodiment. In this embodiment, the high driving voltage V_(GH)of the selection line is 10V, and the low driving voltage V_(GL) of theselection line is 0V. Subsequently, the high voltage V_(X1) and lowvoltage V′_(X1) of the first node X₁ is between V_(GH) and V′_(GH),wherein the V_(X1) is higher than V′_(X1). The voltage waveform at thefirst node X₁ is at least V_(th) _(—) _(Q1) higher than that of the lowdriving voltage V_(GL), wherein, the V_(th) _(—) _(Q1) is the thresholdlevel of the two-terminal photosensing transistor Q₁.

FIG. 5 is a schematic view of the active photosensing pixel. Thisembodiment is similar with FIG. 3. As such, for simplification, thecircuit and sequence of selection lines will not be described in detailhere. It should be noted that the active photosensing pixel P₂₂ mayfurther comprise a sensitivity control capacitor C_(sensitivity),wherein the sensitivity control capacitor C_(sensitivity) has a firstterminal connecting to the first node X₁ and a second terminalconnecting to the second reference voltage V_(ref2).

Following is the description for the function of the sensitivity controlcapacitor C_(sensitivity). According to the descriptions above, byadjusting the voltage (i.e. V_(X1) corresponding to the first node X₁)of the driving transistor Q₂, it can be determined whether the activephotosensing pixel P₂₂ has received the incident light. Following is therelationship between the voltage differential ΔV_(X1) of first node X₁and the voltage differential ΔV_(Sel) _(—) ₂ of selection line Sel_2, inthis embodiment:

${\Delta \; V_{X\; 1}} = {\frac{C_{sensitivity}}{C_{reset} + C_{sensitivity}}\Delta \; V_{{Sel\_}2}}$

wherein when the diode current I_(diode) is high (meaning ΔV_(X1) isgreatly differed), a sensitivity control capacitor C_(sensitivity) withlarger capacitance is employed, and when a diode current I_(diode) islow (meaning ΔV_(X1) is differed slightly), the sensitivity controlcapacitor C_(sensitivity) with smaller capacitance is employed.Specifically, when the voltage differential ΔV_(Sel) _(—) ₂ is aconstant and a high sensitivity of the driving transistor Q₂ is required(meaning ΔV_(X1) has not greatly differed), the sensitivity controlcapacitor C_(sensitivity) with smaller capacitance will be employed.Therefore, even if the incident light hv is weak, resulting in a lowdiode current I_(diode), the driving transistor Q₂ can still sense theweak incident light hv by employing a sensitivity control capacitorC_(sensitivity) with small capacitance. Thus, compared with theconvention method, this embodiment has a better signal-to-noise ratio(SNR).

FIG. 6 is a schematic view showing the active photosensing array and asequence diagram of the corresponding selection lines. The activephotosensing array M_(photosensing) comprises a plurality of selectionlines Sel_1-Sel_4, a plurality of output lines Out_0-Out_3, a pluralityof active photosensing pixels P₁₁-P₄₃, a driving circuit 50 and asensing circuit 51. In this embodiment, each active photosensing pixelP₁₁-P₄₃ is similar with those of the embodiment in FIG. 5, in general.For simplification, the description of the circuit connection and thesequence diagram of the selection lines Sel_1-Sel_4 will not bedescribed in detail. The driving circuit 50 may enable the selectionsignal to the selection lines Sel_1-Sel-4 in sequence. For example, forthe first cycle T₁ (exposure and readout cycle), the selection lineSel_2 is pulled up to be higher than that of the first node X₁ (i.e.high driving voltage V_(GH)). Subsequently, the two-terminalphotosensing transistor Q₁ is served as a diode and produces the diodecurrent I_(diode)/I′_(diode) according to the intensity of the incidentlight hv, such that the first node X₁ is charged to the high voltageV_(X1)/V′_(X1). When the voltage of the first node X₁ is higher thanthat of the threshold level of the driving transistor Q₂, the latter isconducted and produces an output current to the output line Out_2.Afterwards, the sensing circuit 51, by means of detecting or determiningthe output current, determines whether the active photosensing pixel P₂₂has received the incident light hv. Thus, the first cycle T₁ is also thereadout cycle. The threshold level of the driving transistor Q2 isdesigned to be between the voltages V_(X1) and V′_(X1) corresponding tothe diode current I_(diode) of the incident light and the diode currentI′_(diode) having no incident respectively. Hence, the drivingtransistor Q2 is turned on under the circumstance that the first node X₁is V_(X1), and is not turned on while the first node X₁ is V′_(X1).

Furthermore, for the second cycle, the voltage level of the selectionline Sel_2 is pulled down to be lower than that of the first node X₁(such as the low driving voltage V_(GL)), and the driving transistor Q₂resets the latter by resetting the reset capacitor C_(reset) to turn offthe driving transistor Q₂. Note that when the selection line Sel_2 ispulled high under the first cycle T₁, the selection lines Sel_1, Sel_3and Sel_4 will be pulled low under the second cycle T2. In other words,the scan array corresponding to the selection lines Sel_1, Sel_3 andSel_4 will be turned off, so that the neighboring selection lines Sel_1and Sel_3 of the selection line Sel_2 will not interrupt the selectionline Sel_2. In other embodiments, the active photosensing arrayM_(photosensing) includes at least four selection lines, above threescan lines and more than twelve photosensing pixels. Persons skilled inthe art may design the active photosensing array M_(photosensing) inaccordance with product needs.

FIG. 7 is a schematic view showing a display with an active photosensingarray and a sequence diagram corresponding to the selection lines. Thedisplay with the active photosensing array M_(sensing-display) comprisesa plurality of selection lines Sel_1-Sel_4, a plurality of output linesOut_0-Out_3, a plurality of data lines data_1-data_4, a plurality ofactive photosensing and displaying units U₁₁-U₄₃, a driving circuit 50,a sensing circuit 51 and a data driving circuit 52. As shown in FIG. 7,each of the active photosensing and displaying units U₁₁-U₄₃ comprisesan active photosensing pixel (as P₂₂) and a display pixel (as S₂₂), andthereamong, each of the active photosensing pixels can be installed inaccordance with the above-said embodiments. Therefore, detaileddescriptions of the active photosensing pixel and display pixel will beomitted here for brevity. With reference to the display pixel S₂₂, itcomprises a switch transistor Q₃ and a liquid crystal capacitor C_(LC),and thereamong, the switch transistor Q₃ comprises a first terminalcoupled to the second data line data_2, a second terminal, and a controlterminal coupled to the selection line Sel_2. Meanwhile, the liquidcrystal capacitor C_(LC) comprises a first terminal coupled to thesecond terminal of the switch transistor Q3 and a second terminalcoupled to a third reference voltage V_(ref3).

The following is the discussion for the operation of the display withthe active photosensing array. For example, for the first cycle, thevoltage level of the selection line Sel_1 is pulled up to be higher thanthat of the first node X₁ of the photosensing pixel P₂₂ (i.e. the highdriving voltage V_(GH)). Subsequently, the two-terminal photosensingtransistor Q₁ is served as a diode and produces the diode currentI_(diode) according to the extent of the incident light hv, so that thefirst node X₁ is charged to a high voltage V_(X1)/V′_(X1). When thevoltage of the first node X₁ is higher than that of the threshold levelof the driving transistor Q₂ voltage, the driving transistor Q₂ isturned on and produces an output current to the output line Out_2, andthen, the sensing circuit 51, by means of detecting or determining theoutput current, determines whether the active photosensing pixel P₂₂receives the incident light hv. Therefore, the first cycle T₁ is alsothe readout cycle. The threshold level of the driving transistor Q2voltage is designed to be between the voltages V_(X1) and V′_(X1)corresponding to the diode current I_(diode) of the incident light andthe diode current I′_(diode) having no incident light, respectively.Hence, the driving transistor Q2 is turned on when the voltage level ofthe first node X₁ is V_(X1) and turned off when the voltage level of thefirst node X₁ is V′_(X1).

Subsequently, during the second cycle T₂, the voltage of the selectionline Sel_1 is pulled clown to be lower than that of the first node X₁(such as the low driving voltage V_(GL)), and the reset capacitorC_(reset) will reset the first node voltage V_(X1), due to thecapacitive coupling effect, to turn off the driving transistor Q₂. Inaddition, the driving circuit 50 enables the selection line Sel_2, sothat the switch transistor Q3 of the display pixel S₂₂ is turned on.Therefore, the display pixel S₂₂ displays images according to the datareceived from the second data line data_2.

The embodiments provide an active photosensing pixel and photosensingmethod. Compared with the conventional passive photosensing pixel, thephotosensing pixels in the embodiments have higher signal-to-noise ratioand driving ability, which can meet the needs of large displaydimensions. Moreover, the control terminal of the two-terminalphotosensing transistor Q₁ is connected to the second terminal;therefore, variations in threshold level voltages of the two-terminalphotosensing transistor Q₁ will not influence the display device. In theembodiments, the photosensing pixels and array can be disposed at thebackplane of the display device and replace the conventional chargecoupled device (CCD) photo sensor and the CMOS photo sensor.

While the invention has been described by way of example and in terms ofthe embodiments, it is to be understood that the invention is notlimited to the disclosed embodiments. To the contrary, it is intended tocover various modifications and similar arrangements (as would beapparent to those skilled in the art). Therefore, the scope of theappended claims should be accorded the broadest interpretation so as toencompass all such modifications and similar arrangements.

1. An active photosensing pixel, comprising: a two-terminal photosensingtransistor, having a first terminal coupled to a first node, a secondterminal coupled to a selection line and a control terminal connected tothe second terminal; a driving transistor, having a first terminalcoupled to a first reference voltage, a second terminal coupled to anoutput line and a control terminal connected to the first node; and areset capacitor, having a first terminal connected to the controlterminal of the two-terminal photosensing transistor, and a secondterminal connected to the first node.
 2. The active photosensing pixelas claimed in claim 1, further comprising a sensitivity controlcapacitor, having a first terminal connected to the first node and asecond terminal connected to a second reference voltage.
 3. The activephotosensing pixel as claimed in claim 1, wherein, during an exposureand readout cycle, a level of the selection line is pulled up to behigher than that of the first node, and the two-terminal photosensingtransistor is served as a diode to produce a diode current according toan incident light, wherein the first node is charged by the diodecurrent, such that the driving transistor generates an output current tothe output line.
 4. The active photosensing pixel as claimed in claim 3,wherein, during a reset cycle after the exposure and readout cycle iscompleted, the level of the selection line is pulled down to be lowerthan that of the first node, such that the reset capacitor resets thefirst node to turn off the driving transistor due to the capacitivecoupling effect.
 5. The active photosensing pixel as claimed in claim 1,wherein the two-photosensing transistor is an N-type a-Si:H TFT.
 6. Anactive photosensing array, comprising: a plurality of selection lines; aplurality of output lines; and a plurality of active photosensingpixels, each including: a two-terminal photosensing transistor, having afirst terminal coupled to a first node, a second terminal coupled to acorresponding selection line and a control terminal connected to thesecond terminal; a driving transistor, having a first terminal coupledto a first reference voltage, a second terminal coupled to acorresponding output line and a control terminal connected to the firstnode; and a reset capacitor, having a first terminal connected to thecontrol terminal of the two-terminal photosensing transistor, and asecond terminal connected to the first node.
 7. The active photosensingarray as claimed in claim 6, wherein the active photosensing pixels eachcomprise a sensitivity control capacitor having a first terminalconnected to the first node and a second terminal connected to a secondreference voltage.
 8. The active photosensing, array as claimed in claim6, wherein, during an exposure and readout cycle, a level of thecorresponding selection line is pulled up to be higher than that of thelevel at the first node, and the two-terminal photosensing transistor isserved as a diode to produce a diode current according to an incidentlight, wherein the first node is charged by the diode current, such thatthe driving transistor generates an output current to the correspondingoutput line.
 9. The active photosensing array as claimed in claim 8,wherein during a reset cycle after the exposure and read out cycle iscompleted, the level of the corresponding selection line is pulled downto be lower than that of the first node, such that the reset capacitorresets the first node to turn off the driving transistor due to thecapacitive coupling effect.
 10. The active photosensing array as claimedin claim 6, wherein the two-terminal photosensing transistor is anN-type a-Si:H TFT.
 11. A photosensing method for an active photosensingpixel, wherein the active photosensing pixel comprises a two-terminalphotosensing transistor having a first terminal coupled to a first node,a second terminal coupled to a selection line and a control terminalconnected to the second terminal, a driving transistor having a firstterminal coupled to a first reference voltage, a second terminal coupledto an output line and a control terminal connected to the first node,and a reset capacitor, having a first terminal connected to the controlterminal of the two-terminal photosensing transistor, and a secondterminal connected to the first node, comprising: providing a firstvoltage to the selection line, during an exposure and readout cycle,such that the two-terminal photosensing transistor is served as a diode,and generating a diode current to charge the first node when thetwo-terminal transistor receives an incident light, such that thedriving transistor is turned on according to a voltage level of thefirst node to produce an output current to the selection line.
 12. Thephotosensing method as claimed in claim 11, further comprising providinga second voltage to the selection line during a reset cycle after theexposure and readout cycle is completed, such that the reset capacitorresets the first node to turn off the driving transistor.
 13. Thephotosensing method as claimed in claim 11, wherein the first voltage ishigher than that of the level of the first node.
 14. The photosensingmethod as claimed in claim 12, wherein the second voltage is lower thanthat of the first node.
 15. The photosensing method as claimed in claim11, wherein the two-terminal photosensing transistor is an N-type a-Si:HTFT